Problems with 1981 Seca xj750

As I have said before, this sounds like the engine cam timing is off and the timing is late as a result.

 

Polock - hete are two pictures of cylinder #1 in TDC.

 

It doesn't look like the pointer has ever been disturbed, that's the factory paint mark.

 

But yes, that does sound like what is going on, if the compression really is 150psi and it is truly spitting back thru the carbs...............only one way for that to happen.

 

Seems worthwhile.

Just to throw something else out there. a car with a plugged exhaust system will barely idle. Is it possible for the supertrapp exhaust on an XJ to have that severe of a block in the exhaust?

 

Sure it is. Mice will build a nest in whatever hole thay can get to.

 

Pull the exhaust system and check it out........

 

Didn't this bike already run for a few hundred miles or so?

 

Yes it did. I doubt that there is an obstruction myself.

 

The bike actually ran for 1400 miles.

I was going to take off the exhaust and check for mice anyway but after looking at the pipe concluded the mouse theory would be impossible based on the super trap...

 

Perhaps removing the end cap and the disks while trying to start might tell you if there is a restriction.

 

Would sea foam gum up the exhaust flow or the disk gap? Oh my, this thread is intriguing so far...

 

No, Seafoam won't hurt the exhaust, not even a Supertrapp........

 

But would stutter above 70 and progressively got worse.

I have never owned a supertrapp so I did some reading. Here is a comment about the packing:

"After removing the core, remove any left over packing. Next, make sure the holes in the perforated tube are clear. Wrap the new packing around the tube. Use masking tape at the top and around the middle to hold the packing in place. Then line up the holes and replace the core."

So, for you guys that know, would you also have to pull the tube to really open the exhaust or just remove the end cap?

 

On the ones I have you just remove the end cap and the disks.

 

Yes, you just pull the cap and discs to remove the core and packing. The fiberglass could have become a problem during the ride. If it hasn't been repacked in quite some time it is possible that it has broken up and caused a restriction.

 

i've dealt with several supertrapps over the years. i really doubt it's plugged up. the packing would have to go thru the perforated pipe then out to the disks. by then it would just blow out.
i will tell ya that there's way too many disks in there, try 5 max

 

the chain is moving from right to left, look at the (what appears to be) the mark on the left cam approaching TDC. the right cam shows the mark definitely past TDC
both appear to be one tooth off
is that what you guys see?

 

Good eye. I hadn't caught that on the first look. It does appear that the cams are out of time.
IIRC the '81 has a manual cam chain adjuster, and if it hadn't been adjusted in a while that could lead to the chain being loose enough that the chin could skip teeth, and that could have happened late in the trip.

 

It takes a village to fix a bike... or at least to teach the new guy how to fix it...

Based on recent comments, it was easy to pull the end cap on the super trap so I did. Some carbon fell out as shown in the picture below but I didn't think much of it. I looked down the inside of the exhaust pipe and it looked clear - a little dirty but no obstruction. With the cap off I decided to hit the starter button just for the heck of it and was startled when it started right up. I rolled on the throttle and the tach jumped up to 4000 almost instantly. I was stunned. I quickly hooked up my auxiliary gas tank and drove around the block. IT'S ALIVE... I'm changing the bike's name from Chucky to Optimus Prime. There was quite a bit of carbon buildup in the end cap and disks. I'm cleaning the disks and cap in carb cleaner tonight.

I feel very grateful to all of you for helping me out. Tomorrow I'll re-swap all the parts from the Widow back to Prime and life is once more good at the Gold house hold...